Lithographic apparatus and device manufacturing method

ABSTRACT

In an immersion lithography apparatus or device manufacturing method, the position of focus of the projected image is changed during imaging to increase focus latitude. In an embodiment, the focus may be varied using the liquid supply system of the immersion lithographic apparatus.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from European patent application EP03256498.1, filed Oct. 15, 2003, which is incorporated herein in itsentirety.

FIELD

The present invention relates to a lithographic apparatus and a devicemanufacturing method.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a target portion of a substrate. Lithographic apparatus can beused, for example, in the manufacture of integrated circuits (ICs). Inthat circumstance, a patterning device, such as a mask, may be used togenerate a circuit pattern corresponding to an individual layer of theIC, and this pattern can be imaged onto a target portion (e.g.comprising part of, one or several dies) on a substrate (e.g. a siliconwafer) that has a layer of radiation-sensitive material (resist). Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively exposed. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion in one go, andso-called scanners, in which each target portion is irradiated byscanning the pattern through the projection beam in a given direction(the “scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction.

It has been proposed to immerse the substrate in the lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g. water, so as to fill a space between the final element ofthe projection system and the substrate. The point of this is to enableimaging of smaller features because the exposure radiation will have ashorter wavelength in the liquid. (The effect of the liquid may also beregarded as increasing the effective NA of the system and alsoincreasing the depth of focus.)

However, submersing the substrate or substrate and substrate table in abath of liquid (see, for example, United States patent U.S. Pat. No.4,509,852, hereby incorporated in its entirety by reference) means thatthere is a large body of liquid that must be accelerated during ascanning exposure. This requires additional or more powerful motors andturbulence in the liquid may lead to undesirable and unpredictableeffects.

One of the solutions proposed is for a liquid supply system to provideliquid on only a localized area of the substrate and in between thefinal element of the projection system and the substrate (the substrategenerally has a larger surface area than the final element of theprojection system). One way which has been proposed to arrange for thisis disclosed in PCT patent application WO 99/49504, hereby incorporatedin its entirety by reference. As illustrated in FIGS. 2 and 3, liquid issupplied by at least one inlet IN onto the substrate, preferably alongthe direction of movement of the substrate relative to the finalelement, and is removed by at least one outlet OUT after having passedunder the projection system. That is, as the substrate is scannedbeneath the element in a −X direction, liquid is supplied at the +X sideof the element and taken up at the −X side. FIG. 2 shows the arrangementschematically in which liquid is supplied via inlet IN and is taken upon the other side of the element by outlet OUT which is connected to alow pressure source. In the illustration of FIG. 2 the liquid issupplied along the direction of movement of the substrate relative tothe final element, though this does not need to be the case. Variousorientations and numbers of in- and out-lets positioned around the finalelement are possible, one example is illustrated in FIG. 3 in which foursets of an inlet with an outlet on either side are provided in a regularpattern around the final element.

Contrast changes in an imaged pattern may be a problem in lithographicapparatus. Contrast changes can occur as a result of laser band widthvariations, changes in levels of stray light, etc. Further, it oftendesirable to increase the focus latitude (the allowed variation inprojection beam position of focus to obtain an imaged pattern withallowable resolution) in a lithographic apparatus.

SUMMARY

Accordingly, it would be advantageous, for example, to increase focuslatitude and/or stabilize contrast control in immersion lithography.

According to an aspect of the invention, there is provided alithographic apparatus, comprising:

an illumination system configured to condition a beam of radiation;

a support structure configured to hold a patterning device, thepatterning device configured to impart the beam with a pattern in itscross-section;

a substrate table configured to hold a substrate;

a projection system configured to project the patterned beam onto atarget portion of the substrate;

a liquid supply system configured to fill a space between the projectionsystem and the substrate with a liquid; and

a beam controller configured to focus the patterned beam at a pluralityof positions relative to the substrate during projection of thepatterned beam.

By focusing the patterned beam at a plurality of positions relative tothe substrate during projection of the patterned beam, focus latitudemay be increased for a fixed patterning device feature size so that theprocess window may be increased. By focusing the patterned beam at aplurality of positions (by, for example, varying the position of focus)during projecting of the patterned beam, throughput may not bedeleteriously effected. With the same apparatus and/or method, thecontrast control of the apparatus may be stabilized.

In an embodiment, the beam controller is configured to vary the positionof focus by moving a final element of the projection system. In animmersion lithographic apparatus, the final element of the projectionsystem is often a plane parallel plate which is used to seal theprojection optics from the immersion liquid. By tilting the finalelement relative to other optical elements in the projection systemand/or moving the final element in a direction substantially parallel tothe optical axis of the apparatus, the position of the focus can easilybe varied. This movement can be achieved by one or more actuators (apiezoelectric actuator may be suitable) or, in an embodiment, the finalelement is movable by action of pressure of the liquid. Using pressureof the liquid may make the projection system simple in design and theliquid supply system perform a dual function. The movement using theaction of pressure of the liquid can be achieved by varying the inputpressure of the liquid and/or by arranging that movement of thesubstrate is effective to generate the pressure. So, for example, whenthe substrate is scanned under the projection system, the movement ofthe substrate may be used to generate a force on the final element ofthe projection system through the liquid to vary the position of focus.

Immersion lithography also lends itself to varying the position of focusby using the controller to control the liquid supply system and vary theposition of focus by altering the refractive index of the liquid. Thiscan be achieved by a change in composition of the liquid and/or by achange in temperature of the liquid.

Alternatively or additionally, the position of focus may be varied bymoving the substrate, by broadening the bandwidth of the projection beamand/or tilting the projection beam with respect to the substrate.

According to a further aspect of the invention, there is provided adevice manufacturing method, comprising:

providing a liquid to a space between a projection system of alithographic apparatus and a substrate;

projecting a patterned beam of radiation, using the projection system,through the liquid onto a target portion of the substrate; and

focusing the patterned beam of radiation at a plurality of positionsrelative to the substrate.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,liquid-crystal displays (LCDs), thin-film magnetic heads, etc. Theskilled artisan will appreciate that, in the context of such alternativeapplications, any use of the terms “wafer” or “die” herein may beconsidered as synonymous with the more general terms “substrate” or“target portion”, respectively. The substrate referred to herein may beprocessed, before or after exposure, in for example a track (a tool thattypically applies a layer of resist to a substrate and develops theexposed resist) or a metrology or inspection tool. Where applicable, thedisclosure herein may be applied to such and other substrate processingtools. Further, the substrate may be processed more than once, forexample in order to create a multi-layer IC, so that the term substrateused herein may also refer to a substrate that already contains multipleprocessed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of 365, 248, 193, 157 or 126 nm).

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a projection beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the projection beam may not exactly correspond to thedesired pattern in the target portion of the substrate. Generally, thepattern imparted to the projection beam will correspond to a particularfunctional layer in a device being created in the target portion, suchas an integrated circuit.

A patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions; in this manner, thereflected beam is patterned. In each example of a patterning device, thesupport structure may be a frame or table, for example, which may befixed or movable as required and which may ensure that the patterningdevice is at a desired position, for example with respect to theprojection system. Any use of the terms “reticle” or “mask” herein maybe considered synonymous with the more general term “patterning device”.

The term “projection system” used herein should be broadly interpretedas encompassing various types of projection system, including refractiveoptical systems, reflective optical systems, and catadioptric opticalsystems, as appropriate for example for the exposure radiation beingused, or for other factors such as the use of an immersion fluid or theuse of a vacuum. Any use of the term “projection lens” herein may beconsidered as synonymous with the more general term “projection system.”

The illumination system may also encompass various types of opticalcomponents, including refractive, reflective, and catadioptric opticalcomponents for directing, shaping, or controlling the projection beam ofradiation, and such components may also be referred to below,collectively or singularly, as a “lens.”

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 2 illustrates, in cross-section, a liquid supply system accordingto an embodiment of the invention;

FIG. 3 depicts the liquid supply system of FIG. 2 in plan;

FIG. 4 illustrates a first embodiment of the present invention;

FIG. 5 illustrates a second embodiment of the present invention;

FIG. 6 illustrates a third embodiment of the present invention; and

FIG. 7 illustrates a fifth embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to aparticular embodiment of the invention. The apparatus comprises:

an illumination system (illuminator) IL for providing a projection beamPB of radiation (e.g. UV radiation).

a first support structure (e.g. a mask table) MT for supporting apatterning device (e.g. a mask) MA and connected to a first positioningdevice PM for accurately positioning the patterning device with respectto item PL;

a substrate table (e.g. a wafer table) WT for holding a substrate (e.g.a resist-coated wafer) W and connected to a second positioning device PWfor accurately positioning the substrate with respect to item PL; and

a projection system (e.g. a refractive projection lens) PL for imaging apattern imparted to the projection beam PB by the patterning device MAonto a target portion C (e.g. comprising one or more dies) of thesubstrate W.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above).

The illuminator IL receives a beam of radiation from a radiation sourceSO. The source and the lithographic apparatus may be separate entities,for example when the source is an excimer laser. In such cases, thesource is not considered to form part of the lithographic apparatus andthe radiation beam is passed from the source SO to the illuminator ILwith the aid of a beam delivery system BD comprising for examplesuitable directing mirrors and/or a beam expander. In other cases thesource may be integral part of the apparatus, for example when thesource is a mercury lamp. The source SO and the illuminator IL, togetherwith the beam delivery system BD if required, may be referred to as aradiation system.

The illuminator IL may comprise adjusting means AM for adjusting theangular intensity distribution of the beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator ILgenerally comprises various other components, such as an integrator INand a condenser CO. The illuminator provides a conditioned beam ofradiation, referred to as the projection beam PB, having a desireduniformity and intensity distribution in its cross-section.

The projection beam PB is incident on the mask MA, which is held on themask table MT. Having traversed the mask MA, the projection beam PBpasses through the projection system PL, which focuses the beam onto atarget portion C of the substrate W. With the aid of the secondpositioning device PW and position sensor IF (e.g. an interferometricdevice), the substrate table WT can be moved accurately, e.g. so as toposition different target portions C in the path of the beam PB.Similarly, the first positioning device PM and another position sensor(which is not explicitly depicted in FIG. 1) can be used to accuratelyposition the mask MA with respect to the path of the beam PB, e.g. aftermechanical retrieval from a mask library, or during a scan. In general,movement of the object tables MT and WT will be realized with the aid ofa long-stroke module (coarse positioning) and a short-stroke module(fine positioning), which form part of the positioning devices PM andPW. However, in the case of a stepper (as opposed to a scanner) the masktable MT may be connected to a short stroke actuator only, or may befixed. Mask MA and substrate W may be aligned using mask alignment marksM1, M2 and substrate alignment marks P1, P2.

The depicted apparatus can be used in the following preferred modes:

1. In step mode, the mask table MT and the substrate table WT are keptessentially stationary, while an entire pattern imparted to theprojection beam is projected onto a target portion C in one go (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the mask table MT and the substrate table WT arescanned synchronously while a pattern imparted to the projection beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the masktable MT is determined by the (de-)magnification and image reversalcharacteristics of the projection system PL. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the mask table MT is kept essentially stationaryholding a programmable patterning device, and the substrate table WT ismoved or scanned while a pattern imparted to the projection beam isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning device isupdated as required after each movement of the substrate table WT or inbetween successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizes aprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

Another immersion lithography solution with a localized liquid supplysystem solution which has been proposed is to provide the liquid supplysystem with a seal member which extends along at least a part of aboundary of the space between the final element of the projection systemand the substrate table. The seal member is substantially stationaryrelative to the projection system in the XY plane though there may besome relative movement in the Z direction (in the direction of theoptical axis). A seal is formed between the seal member and the surfaceof the substrate. In an embodiment, the seal is a contactless seal suchas a gas seal. Such a system with a gas seal is disclosed in UnitedStates patent application no. U.S. Ser. No. 10/705,783, herebyincorporated in its entirety by reference.

While one or more embodiments of the invention are described in relationto increasing the focus latitude of the lithographic apparatus, one ormore embodiments of the invention may equally be applicable tostabilizing the contrast of the lithographic apparatus.

To increase the focus latitude (although possibly at the expense of doselatitude), a technique called “focus drilling” has been proposed. In aform of focus drilling, the pattern is exposed a number of times on thesurface of the substrate, each time with the pattern in a differentfocal plane. This may increase the depth of focus achievable for certainfeature types. Although this “focus drilling” method may increase thefocus latitude, it may be at the expense of a decrease in throughput aseach image needs to be exposed multiple times.

It is possible to increase the focus latitude of a lithographicapparatus by focusing the projected pattern at a plurality of locationsduring scanning. A way of doing this in an immersion lithographicapparatus is illustrated in FIG. 4. Any method of filling a spacebetween the final element 50 of the projection system PL and thesubstrate W may be employed. In FIG. 4, a liquid confinement system LCS(of a liquid supply system), such as disclosed in United States patentapplication U.S. Ser. No. 10/705,783, is used for illustration purposes.

FIG. 4 a shows the projection beam PB passing through one or more lensesof the projection system PL and through the final element 50 of theprojection system which is illustrated as a so-called “Abschlussplatte”.The Abschlussplatte is a plane parallel plate which is used to seal theprojection system PL from the immersion liquid 10. Other kinds of finalelement 50 may used including, for example, a lens.

In FIG. 4 a, the final element 50 of the projection system PL ispositioned orthogonally to the projection beam PB and the optical axisof the apparatus. If the final element 50 is tilted with respect to theoptical axis of the apparatus, as is illustrated in FIGS. 4 b and 4 c,the angle of impingement of the projection beam PB on the substrate Wcan be varied away from orthogonal by a few 100 μrads. In this way, whenthe substrate W is moved under the projection system PL during imaging(in a direction parallel to the top surface of the substrate), acontinuous focus shift is experienced by every point on the substrate W.This is illustrated schematically in FIG. 4 d and the focus shift isillustrated by arrows 30.

The final element 50 may be tilted using one or more piezoelectricactuators or other forms of actuator (active actuation). Conveniently inan immersion lithographic apparatus use may be made of a force generatedin the immersion liquid 10 by movement of the substrate W under theprojection system PL during imaging to move the final element 50(passive actuation). This may be achievable by mounting the finalelement to the remainder of the projection system with a correctrigidity (which can be established by trial and error). In FIG. 4 b, thesubstrate W is moving to the left as illustrated. This creates abuild-up in pressure in the immersion liquid 10 on the left-hand side.If the projection system PL is assembled in the correct way, the extrabuild-up in pressure in the immersion liquid 10 on the left-hand side ofthe final element 50 may be effective to tilt the final element 50 awayfrom its orthogonal relationship to the optical axis of the projectionsystem. By tilting the final element 50 away from the optical axis, theprojection beam PB exiting the final element 50 is also tilted ordeflected away from the optical axis in the direction of movement of thesubstrate W. As is illustrated in FIG. 4 d the tilting of the projectionbeam PB relative to the substrate W results in a continuous focus shiftexperienced by every point on the substrate. The amount of tilt neededis about 1 to 2 μm every 10 mm.

FIG. 4 c illustrates the situation when the substrate W moves towardsthe right. In this case the pressure built-up in the immersion liquid 10is also on the right-hand side as illustrated and so the final element50 of the projection system PL is tilted in the opposite direction tothat illustrated in FIG. 4 b. The tilting of the final element 50 in theopposite direction results in the projection beam PB also being tiltedin the opposite direction so that the introduced focus offset is in thedirection of the scanning substrate W and a “focus drilling” can beachieved.

Tilting of the final element 50, if it is a plane parallel plate, wouldnot have the effect of causing “focus drilling” in a non-immersionlithographic apparatus—it is the large optical path differenceachievable with a refractive index larger than 1 which helps ensure thechange in position of focus. The immersion liquid, when the finalelement is tilted, is like a wedge shaped optical element.

A similar effect may be achieved by moving the final element 50 in adirection substantially parallel to an optical axis of the lithographicapparatus. However, in such a case it may not be possible tocontinuously scan the substrate W. Several discrete images may need tobe projected onto the substrate W at different focus levels. Thedifferent focus levels are achieved by varying the vertical height ofthe final element 50 and thus the amount of liquid between the finalelement 50 and the substrate W. Because the refractive index of theliquid is significantly greater than 1, the position of focus of theprojection beam PB varies.

A second embodiment is illustrated schematically in FIG. 5. As with thefirst embodiment, the projection beam PB is tilted relative to thesubstrate W so that the projection beam does not project orthogonallyonto the substrate surface. In the case illustrated in FIG. 5, this isachieved by tilting the substrate W relative to the projection system PLoptical axis so that a continuous focus shift is experienced by everypoint on the substrate W as the substrate W is scanned under theprojection system PL and the mask MA is scanned in the oppositedirection above the projection system PL. During scanning, the targetportion of the substrate W (which is changing in position on thesubstrate) remains a constant distance from the final element of theprojection system PL.

The tilting of the substrate W by an amount of about 1 μm difference invertical height between ends of the substrate, as is shown schematicallyin FIG. 5, may be achieved in an immersion lithographic apparatus.Indeed, the liquid supply systems described herein may work particularlywell in this embodiment.

A third embodiment is illustrated in FIG. 6 and is the same as the firstembodiment except as described below. In the third embodiment, theprojection beam PB is maintained, as usual, on the optical axis of theprojection system PL. However, in order to change the position of focusof the projection beam PB relative to the substrate W, the substrate Wis moved up and down by about 1 μm in a direction substantially parallelto the optical axis as illustrated. This may be carried out duringexposure of the substrate W wherein the substrate W is vibrated about 1μm per slit scan at a frequency of a few cycles per exposure to lessthan the laser frequency (1-4 kH_(z)). Alternatively, multipleexposures, one at each position, may be carried out.

Fourth and fifth embodiments are the same as the first embodiment exceptit is the characteristics of the immersion liquid 10 which are varied inorder to achieve the desired variation in position of focus of thepatterned projection beam PB. As with the other embodiments, any liquidsupply system may be used, though for the fourth embodiment, a liquidsupply system, such as that illustrated in FIGS. 2 and 3, may be mostsuitable. For the fifth embodiment, a liquid confinement system LCS,such as that described in United States patent application U.S. Ser. No.10/705,783, may be optimal.

In the fourth and fifth embodiments, the refractive index of theimmersion liquid 10 is varied with time. In particular, when thesubstrate W is scanned under the projection system PL during exposurethe refractive index of the immersion liquid 10 is deliberately changedin the horizontal direction. One way of doing this, using the apparatusof FIG. 2, is to change the composition of liquid applied to the surfaceof the substrate W through inlet IN with time, for example, by using acontinually varying binary composition of two liquids with differentrefractive indexes. This variation in refractive index will have theeffect of varying the position of focus of the projected image relativeto the substrate W to increase the focus latitude.

According the fifth embodiment as described with reference to FIG. 7,the variation of refractive index described above is also possible byvarying the temperature of the immersion liquid. A heater 70 is providedon the liquid confinement system LCS or may be provided somewhere elsein the liquid supply system. The heater 70 is effective to varytemperature in the horizontal direction, i.e. substantially parallel tothe plane of the surface of the substrate W to be imaged duringscanning. This variation in temperature has an effect of changing therefractive index of the immersion liquid 10 such that the position offocus of the projection beam PB relative to the substrate W varies asthe substrate W is scanned under the projection system PL.

The systems of the fourth and fifth embodiments can be used on acontinual scanning basis or for discrete multiple exposures at eachdifferent focus setting. The control can be feed forward or feedback bymeasuring the immersion liquid properties in the downstream or upstreamdirection.

Further ways of focusing the projection beam PB at a plurality ofpositions include varying the bandwidth of the projection beam PB bychanging, for example, the composition of the gas in the lasergenerating the projection beam PB, changing the pressure of gas in thelaser or by using a line narrowing unit. If refractive projection opticsare used, the position of focus depends on the wavelength used (thelenses are said to have axial color). Thus, by increasing the bandwidthof the projection beam PB, multiple aerial images (one per wavelength)are imaged simultaneously. The same effect may be achieved by utilizinga plurality of projection beams of two or more different (substantiallyconstant) wavelengths. The projection beams can be provided by the samesource or different sources. The position of focus of the projectionbeam can also be varied by displacement, tilting or rotation of themask.

It may also be possible to use a birefringent optical element (e.g., alens) in the projection system PL such as a CaF₂ optical element. With abirefringent optical element, it is possible to image the substrate W atvarious positions of focus of the projection beam at the same time.

It will be appreciated that all of the above methods employ precisecontrol of the variables to ensure that the correct amount of variationof position of focus is achieved. For this purpose, a beam controller isprovided.

Any liquid supply system, whether a localized solution or not, can beused according to the one or more embodiments of the present invention.A liquid supply system may be any mechanism that provides a liquid to aspace between the projection system and the substrate and/or substratetable. It may comprise any combination of one or more structures, one ormore liquid inlets, one or more gas inlets, one or more gas outlets,and/or one or more liquid outlets, the combination providing andconfining the liquid to the space. In an embodiment, a surface of thespace may be limited to a portion of the substrate and/or substratetable, a surface of the space may completely cover a surface of thesubstrate and/or substrate table, or the space may envelop the substrateand/or substrate table.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The description is not intended to limit theinvention.

1. A lithographic apparatus, comprising: an illumination systemconfigured to condition a beam of radiation; a support structureconfigured to hold a patterning device, the patterning device configuredto impart the beam with a pattern in its cross-section; a substratetable configured to hold a substrate; a projection system configured toproject the patterned beam onto a target portion of the substrate; aliquid supply system configured to fill a space between the projectionsystem and the substrate with a liquid; and a beam controller configuredto focus the patterned beam at a plurality of positions relative to thesubstrate during projection of the patterned beam.
 2. The apparatusaccording to claim 1, wherein the patterned beam is focused at theplurality of positions by action of the immersion liquid.
 3. Theapparatus according to claim 1, wherein the beam controller isconfigured to vary the position of focus by moving a final element ofthe projection system.
 4. The apparatus according to claim 3, whereinthe final element is movable by action of pressure of the liquid.
 5. Theapparatus according to claim 4, wherein movement of the substrate iseffective to generate the pressure.
 6. The apparatus according to claim3, wherein the final element is tilted relative to other opticalelements in the projection system.
 7. The apparatus according to claim3, wherein the final element is moved in a direction substantiallyparallel to an optical axis of the apparatus.
 8. The apparatus accordingto claim 1, wherein the beam controller is configured to control theliquid supply system to vary the position by altering the refractiveindex of the liquid.
 9. The apparatus according to claim 8, wherein therefractive index is altered by a change in composition of the liquid.10. The apparatus according to claim 8, wherein the refractive index isaltered by a change in temperature of the liquid.
 11. The apparatusaccording to claim 1, wherein the beam controller is configured to varythe position of focus by moving the substrate, by broadening a bandwidthof the beam of radiation, by providing a further beam of radiation of adifferent wavelength to the patterned beam, by tilting the patternedbeam with respect to the substrate, by moving the patterning device, orby any combination of the foregoing.
 12. A device manufacturing method,comprising: providing a liquid to a space between a projection system ofa lithographic apparatus and a substrate; projecting a patterned beam ofradiation, using the projection system, through the liquid onto a targetportion of the substrate; and focusing the patterned beam of radiationat a plurality of positions relative to the substrate.
 13. The methodaccording to claim 12, comprising focusing the patterned beam ofradiation at the plurality of positions by action of the liquid.
 14. Themethod according to claim 12, comprising focusing the patterned beam ofradiation at the plurality of positions by movement of a final elementof the projection system.
 15. The method according to claim 14,comprising moving the final element by action of pressure of the liquidon the final element.
 16. The method according to claim 15, comprisinggenerating the pressure in the liquid by movement of the substrate. 17.The method according to claim 14, comprising tilting the final elementrelative to other optical elements in the projection system.
 18. Themethod according to claim 14, comprising moving the final element in adirection substantially parallel to an optical axis of the lithographicapparatus.
 19. The method according to claim 12, comprising focusing thepatterned beam of radiation at the plurality of positions by alteringthe refractive index of the liquid.
 20. The method according to claim19, wherein altering the refractive index of the liquid compriseschanging the composition of the liquid.
 21. The method according toclaim 20, wherein altering the refractive index of the liquid compriseschanging the temperature of the liquid.
 22. The method according toclaim 12, comprising focusing the patterned beam of radiation at theplurality of positions by moving the substrate, by broadening abandwidth of the patterned beam of radiation, by providing a furtherbeam of radiation of a different wavelength to the patterned beam ofradiation, by tilting the patterned beam of radiation with respect tothe substrate, by moving the patterning device, or by any combination ofthe foregoing.